This invention generally relates to power meshes, and more specifically relates to a power mesh design that includes a bridge for connecting a rail on one layer to a trunk on another layer.
In a four layer circuit design (wherein the layers are M1, M2, M3 and M4), if a standard row of cells is under the M3 horizontal mesh, it is impossible for one of the rails (either Vss or Vdd) on M1 to connect (through vias) up to the M4 portion of the mesh. Shadowing of the rail on M1 effectively denies access to the M4 truck. Hence, cells are left unpowered.
This disadvantage is illustrated in FIG. 1. FIG. 1 illustrates portions of a four layer design (wherein the layers are M1, M2, M3 and M4). A Vdd rail 12 is disposed on M1, a Vss trunk 14 is disposed on M3, and three Vdd trunks 16 are disposed on M4. As shown, because the Vss trunk 14 which is disposed on M3 shadows the Vdd rail 12 on M1, access of the Vdd trunks 16 on M4 to the Vdd rail 12 on M1 is denied. Hence, cells on M4 are left unpowered. It would be advantageous to provide power to cells on one layer (such as M4 in a four layer mesh design) using a rail on another layer (such as a rail on M1 in a four layer mesh design), even though the rail is shadowed by a trunk on a layer between the two layers.
A general object of an embodiment of the present invention is to provide a multiple layer mesh design which provides that cells on one layer are powered using a rail on another layer, even though the rail is shadowed by a trunk on a layer between the two layers.
Another object of an embodiment of the present invention is to provide a multiple layer mesh design which provides a bridge from a rail disposed on one layer to a trunk disposed on another layer.
Still another object of an embodiment of the present invention is to provide a four layer mesh design which provides a bridge associated with a second layer, where the bridge effectively connects a rail on a first layer to a trunk on a fourth layer even though a trunk on the third layer at least partially overshadows the rail on the first layer.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a multiple layer mesh design that includes a first layer having at least one rail, a second layer having at least one bridge associated therewith, a third layer having a trunk, where the trunk at least partially shadows the at least one rail on the first layer, and a fourth layer having at least one trunk, where the at least one bridge associated with the second layer connects the at least one rail on the first layer to the at least one trunk on the fourth layer. If the trunk on the third layer shadows a plurality of rails on the first layer, preferably the width of the at least one bridge is generally equal to a sum of the widths of the rails on the first layer which are shadowed by the trunk on the third layer. If the trunk on the third layer shadows a single rail on the first layer, preferably the width of the at least one bridge is at least twice the width of the rail on the first layer. Preferably, the bridge includes an upper overhang which is dependent upon a number of vias which are on the fourth layer, the third layer and the second layer. Preferably, the bridge includes a lower overhang which is generally equal to a width of the at least one rail which is on the first layer. Preferably, a via population from the trunk on the fourth layer to the at least one bridge associated with the second layer is 100%, and a via population from the at least one bridge associated with the second layer to the at least one rail on the to first layer is 100%. The multiple layer mesh design may include four layers.